Steam turbines are commonly used to drive electrical generators in power plants. A typical steam turbine is a massive yet intricate piece of machinery that must be started up in a controlled manner in order to protect the many turbine components from damage from stresses and distortion that would result from uncontrolled exposure to high temperature and high pressure steam. One part of the turbine startup process is the pre-warming procedure, which includes turbine rotor pre-warming. The rotor core is the massive cylindrical shaft of the turbine to which the steam buckets are attached. The goal of the rotor pre-warming process is to raise rotor core temperatures without exceeding allowable rotor stress limits; after warming, the turbine can be safely accelerated to its nominal operating speed.
Warming of the rotor core is often a limiting factor in the time required to place a turbine in service. In the conventional prewarming process, small amounts of steam are admitted to the high pressure side of the turbine (that is, the turbine blade area) to cause the turbine rotor to warm up, both through direct exposure to the steam and conduction of heat through the metal of the rotor shaft. During the heating process, condensate from the steam admitted to the turbine must be drained off to avoid buildup of liquid in the turbine casing (or shell) and subsequent erosion or cavitation damage to turbine buckets or nozzles. This procedure is continued until the turbine core temperature passes the critical temperature (typically about 350.degree. F.), at which time the turbine is ready to be accelerated and loaded. Cooling of the turbine can present similar problems with respect to inducing thermal stress on the rotor shaft.
It is desirable from an operational standpoint to complete the warm-up or cool down procedures in the shortest time consistent with turbine limitations such as allowable rotor stress. Rapid warm up allows the turbine to be used to meet unplanned short term emergent loads or the like, increasing the efficiency and flexibility of the power generating station of which the steam turbine is a part.
An object of one embodiment of this invention is to provide a steam turbine having a system to reduce thermal stress in warm-up or cool down procedures.